Crosses between the Drosophila paulistorum semispecies produce fertile female but sterile male hybrids regardless of the direction of the crosses and in backcrosses, too. An hypothesis is put forward that the hybrid sterility, which is infectious, is the result of discordance between a cytoplasmic symbiont and the genotype of the host. Support for this hypothesis of causal relationship will come from 1) electron microscopy - a mycoplasma-like symbiont has been localized only in the reproductive tract and in the egg cytoplasm; 2) differential centrifugation - to purify the infectious agent; 3) injection experiments - hybrid sterility can be produced via injections of homogenates of sterile males into nonhybrid females which are then crossed to males of their own strain so that one subsequently acquires sterile but nonhybrid sons; 4) microbiological procedures - to grow the mycoplasma-like microorganism in vitro in order to fulfill Koch's postulate. Unique hereditary mating patterns are associated with the propagation of the mycoplasma-like organism, found in the reproductive tracts of the host insect population. It was discovered that Drosophila females prefer to mate with males that are rare among those courting them. We will attempt to purify the pheromone mediating this frequency- dependent selection, using organic extracts (e.g. acetone, which has given us preliminary positive results) of favored males and parts of these males; differently karyotyped males require petroleum ether not acetone, as their appropriate solvent.